Atherosclerosis

Question 1

Describe how world disease burden has changed from 1990 to 2020

Answer 1

From 1990 globally, IHD and Cerebrovasc D have become 1st and 4th causes of death from 5th and 6th in 1990. Partly due to decrease in death from infection.

Question 2

What are risk factors for atherosclerosis?

Answer 2

Modifiable:

1. Smoking
2. Lipid intake - Lowering lipids can be done through making antibody against a molecule called PCSK9 – LDL cholesterol significantly reduced
3. Blood pressure - ABCD line of treatment used (ACE inhibitor, beta blocker, lipid conducting enzyme inhibitor, diuretics)
4. Diabetes – for T2, first line is dietary advice/weight loss, second gastric surgery, then metformin, then sulfonylureas and finally insulin
5. Obesity
6. Sedentary lifestyle

Non-modifiable:

1. Age
2. Sex
3. Genetic predisposition

Question 3

How has treatment of atherosclerosis changed its epidemiology?

Answer 3

1. Increased use of statin treatment has reduced hyperlipidaemia in population
2. Increased antihypertensive treatment has reduced hypertension
3. Increased obesity has led to increased diabetes
4. New improvements in diabetes treatment have doubtful effect on macrovascular disease - insulin and sulfonylureas definitely don’t, metformin may to a slight extent and SGLT2 inhibitors have been shown to have an impact.
5. Changing pathology of coronary thrombosis possibly related to altered risk factors

Question 4

Why is atherosclerosis focal?

Answer 4

Occur at major bifurcations and bends as vortices form here due to the turbulent blood flow. This causes inflammation of arteries and tend to be location of atherogenesis.

Question 5

How does LDL deposition occur?

Answer 5

Low density lipoproteins (LDL) deposit in the subintimal space and binds to matrix proteoglycans

Question 6

Describe progression of atherosclerosis up to preatheroma stage

Answer 6

1. Adaptive thickening of smooth muscle occurs in coronary artery at lesion prone site so enlarged intima.
2. Fat deposition occurs in the intima and macrophages enter this space to phagocytose fat droplets. However, die of the lipid overload and the dead macrophages plus lipids coalesce. This sets up an inflammatory reaction and foam cells formed. This is a Type 2 lesion.
3. Type 3 lesion (preatheroma) - small pools of extracellular lipid formed.

Question 7

Describe progression of atherosclerosis from preatheroma stage to complicated lesion

Answer 7

1. When core of extracellular lipid formed, atheroma formed.
2. This triggers vascular smooth muscle cells and collagen to proliferate and try to create an abscess like reaction to control inflammation. However, overactivation of inflammatory cells means this is broken down and killing of smooth muscle cells plus collagen causes wall to get thinner and mechanically weaker. Fibroatheroma formed.
3. When fibroatheroma cracks apart, thrombus formed. Oversaturation of cholesterol means they form crystals leading to fissure and haematoma formation.

Question 8

Describe natural history of atherosclerosis and what interventions are possible at each stage

Answer 8

1. Normal
2. Intermediate lesion
3. Advanced lesion
   2 and 3: Primary prevention possible (lifestyle changes and risk factor management)
4. Complications (e.g. stenosis, plaque rupture) - clinical intervention needed (Secondary prevention, Catheter based interventions, Revascularisation surgery, Treatment of heart failure)

Question 9

What is the function of vascular endothelial cells?

Answer 9

Barrier function (e.g. to lipoproteins) and leukocyte recruitment.

Question 10

What is the function of platelets?

Answer 10

Thrombus generation plus cytokine and growth factor release.

Question 11

What is the function of monocytes/macrophages?

Answer 11

Foam cell formation, cytokine and growth factor release, major source of free radicals and metalloproteinases.

Question 12

What is the function of vascular smooth muscle cells?

Answer 12

Migration and proliferation, collagen synthesis, remodelling and fibrous cap formation

Question 13

What is the function of T-lymphocytes?

Answer 13

Macrophage activation

Question 14

Describe inflammation in atherosclerosis

Answer 14

CANTOS Trial showed that atherosclerosis has an inflammatory basis. Multiple mechanisms including cholesterol crystal formation connect lipids and inflammation in atherosclerosis.

Question 15

What is the inflammatory basis of atherosclerosis?

Answer 15

In atherosclerosis, the main inflammatory cells are macrophages, which are derived from blood monocytes. Macrophage subtypes are regulated by combinations of transcription factors binding to regulatory sequences on DNA however, not fully understood.

Question 16

What are the 2 main classes of macrophages?

Answer 16

1. Inflammatory macrophages - adapted to kill microorganisms
2. Resident macrophages - normally homeostatic and suppress inflammatory activity. Examples:
   Alveolar resident macrophages - surfactant lipid homeostasis
   Osteoclasts - calcium and phosphate homeostasis
   Spleen - iron homeostasis

Question 17

Describe LDLs

Answer 17

‘Bad’ cholesterol - Synthesised in liver. Carries cholesterol from liver to rest of the body including arteries. Has apoproteins on surface and phospholipid monolayer.

Question 18

Describe HDLs

Answer 18

‘Good’ cholesterol. Carries cholesterol from ‘peripheral tissues’ including arteries back to liver (=“reverse cholesterol transport”).

Question 19

How are LDLs made more toxic?

Answer 19

Due to action of free radicals on LDL leading to oxidation. Form multiple substances. Families of highly inflammatory and toxic forms of LDL found in vessel walls and oxidation means more likely to be modified through other means. Some of the modifications done to cholesterol come from enzymes in vessel wall as well as inflammatory cells.

Question 20

How does modification of subendothelial trapped LDL occur?

Answer 20

LDLs leak through the endothelial barrier by uncertain mechanisms. LDL is trapped by binding to sticky matrix carbohydrates (proteoglycans) in the sub-endothelial layer and becomes susceptible to modification. Best studied modification is oxidation - represents partial burning. LDL becomes oxidatively modified by free radicals. Oxidised LDL is phagocytosed by macrophages and stimulates chronic inflammation.

Question 21

What is familial hyperlipidaemia?

Answer 21

Autosomal genetic disease (main form dominant with gene dosage). Massively elevated cholesterol (>~20 mmol/L). (effective ‘normal’ ~1-5 mmol/L) due to failure to clear LDLs from blood. Xanthomas and early atherosclerosis; if untreated fatal myocardial infarction before age 20 - used to be a paediatric illness but statsins and PCSK9 inhibitors have reduced MIs.

Question 22

Describe implications of LDLR and statins

Answer 22

LDL receptor expression is negatively regulated by intracellular cholesterol. Cholesterol synthesis is also negatively regulated by cellular cholesterol. Led to the discovery of HMG-CoA reductase inhibitors (= “statins”) for lowering plasma cholesterol.

Question 23

What happens in LDLR negative people?

Answer 23

Macrophages accumulate cholesterol. A second LDL receptor exists but not under feedback control in atherosclerotic lesions. Called ‘scavenger receptor’ since they hoover up chemically modified LDL. Are a family of pathogen receptors that ‘accidentally’ bind OxLDL.

Question 24

What are scavenger receptors?

Answer 24

Two known - original function is to bind pathogens. Macrophage scavenger receptor A known as CD204
Binds to oxidised LDL and gram-positive bacteria like Staphylococci & Streptococci. Binds to dead cells.
Macrophage scavenger receptor B is known as CD36. Binds to oxidised LDL and binds to malaria parasites. Binds to dead cells.

Question 25

What are further roles of macrophages within atherosclerotic plaques?

Answer 25

1. Macrophages have oxidative enzymes that can modify native LDL: NADPH Oxidase and myeloperoxidase (bleach)
2. Phagocytose modified lipoproteins, & become foam cells - high lipid content means macrophage can no longer travel in aqueous environment
3. Express cytokine mediators that recruit monocytes - positive feeback loop perpetuates inflammation
4. Express chemo-attractants & growth factors for VSMC
5. Express Proteinases that degrade tissue

Question 26

What cytokines and chemokines do macrophages produce?

Answer 26

Cytokines are protein immune hormones that activate endothelial cell adhesion molecules. IL-1 upregulates vascular cell adhesion molecule 1 (VCAM-1). VCAM-1 mediates tight monocyte binding. Chemokines are small proteins/chemoattractant to monocytes. Monocyte chemotactic protein-1 (MCP-1) produced which binds to a monocyte G-protein coupled receptor CCR2. Atherosclerosis reduced in mice which don’t have these cytokines/chemokines.

Question 27

What chemo-acttractants and growth factors do macrophages express?

Answer 27

Macrophages release complementary protein growth factors that recruit VSMC and stimulate them to proliferate and deposit extracellular matrix. Platelet derived growth factor released - responsible for VSMC chemotaxis, survival and mitosis. Transforming growth factor beta released - responsible for Increased collagen synthesis and matrix deposition.

Question 28

What is the difference between a normal VSMC and atherosclerotic VSMC?

Answer 28

Normal has more contractile filaments and low matrix deposition gene expression. This is contractile. However, PDGF and TGFbeta change it to synthetic, reducing contractile filaments and increasing matrix deposition gene expression.

Question 29

What proteinases do macrophages express?

Answer 29

Metalloproteinases. Family of ~28 homologous enzymes that activate each other by proteolysis. Degrade collagen. Catalytic mechanism based on Zn. Erodes plaque away. Blood coagulation at the site of rupture may lead to an occlusive thrombus and cessation of blood flow.

Question 30

What are characteristics of vulnerable plaques?

Answer 30

1. Large soft eccentric lipid-rich necrotic core
2. Increased VSMC apoptosis
3. Reduced VSMC & collagen content
4. Thin fibrous cap
5. Infiltrate of activated macrophages expressing MMPs

Question 31

How does macrophage apoptosis happen?

Answer 31

OxLDL derived metabolites are toxic eg 7-keto-cholesterol. While macrophage foam cells have protective systems that maintain survival in face of toxic lipid loading, once overwhelmed, die of apoptosis. Release macrophage tissue factors and toxic lipids into the ‘central death zone’ called lipid necrotic core. Thrombogenic and toxic material accumulates, walled off, until plaque rupture causes it to meet blood.

Question 32

What is NFkB?

Answer 32

Transcription Factor Nuclear Factor kappa B. It is a master regulator of inflammation. Activated by numerous inflammatory stimuli (Scavenger receptors, Toll-like receptors, Cytokine receptors e.g. IL-1). Switches on numerous inflammatory genes (Matrix metalloproteinases, Inducible nitric oxide synthase, Interleukin-1).